This invention relates to an improved microstrip directional coupler. In particular, the invention is directed to a microstrip directional coupler which uses inductive loading to improve directivity.
A directional coupler is used to couple a secondary transmission path to a wave travelling in one direction on a primary transmission path. The secondary transmission path normally has two ports, namely a coupled port which receives a small amount of energy from the wave on the primary transmission path, typically 10 to 20 dB less than that in the primary transmission path, and an isolated port which ideally does not receive any of the coupled energy.
Implementation of directional couplers in microstrip transmission line medium has a number of advantages over other media. These include compact size, simple printed circuit board fabrication techniques, ability to be integrated with other circuitry with no additional mechanical construction techniques, and ease of design using analytical or computer-aided design methods.
However, the traditional microstrip coupler has a poor directivity, which is defined as the ratio of desired power at the coupled port to the undesired power at the isolated port. This stems from the fact that the fields in the microstrip medium exist in two different dielectrics i.e. the air and the substrate. This leads to the even mode fields, which are confined to the substrate, being slower than the odd mode fields, which are partially in the air. The even and odd modes thus do not cancel in the reverse direction, leading to poor directivity.
For example, a traditional quarter-wave long coupled line microstrip directional coupler on 0.787 mm thick Tac-Lam TLY5 substrate provides about 15 dB directivity for a 10 dB coupler, about 8 dB directivity for a 20 dB coupler, and about 5 dB for a 30 dB coupler for a 5% operating bandwidth. In many power monitoring applications, such values are unacceptable as they introduce errors in the system performance.
A number of attempts have been made over the last three decades to overcome this limitation of the microstrip couplers. Podell [A. Podell, xe2x80x9cA High Directivity Microstrip Coupler Techniquexe2x80x9d, 1970-MTT-Symposium Digest, May 1970, pp 33-36.] used a wiggly gap in the coupling region between the main and coupled lines to increase the path travelled by the odd modes to achieve better directivity. Such design is empirical and cannot be easily analysed. Sugiura [T. Sugiura, xe2x80x9cAnalysis of Distributed-Lumped Strip Transmission Linesxe2x80x9d, IEEE Tarns. Microwave Theory and Tech., vol.MTT-25, pp. 656-661.] has attempted to analyse a similar technique to slow down the odd mode using xe2x80x9cdistributed-lumpedxe2x80x9d transmission lines. However, only theoretical results for coupled lines were presented.
Other ways to slow down the odd modes include capacitively loading the odd mode at the ends [G. Schaller, xe2x80x9cOptimnisation of Microstrip Directional Couplers with Lumped Capacitorsxe2x80x9d, A.E.U.Vol.31, July-August 1977, pp 301-307.], or at the middle of the coupler by connecting capacitors between the main line and the coupled lines. Although these techniques give good directivity, they require very small values of capacitance (of the order of a fraction of a picofarad). These capacitors can be realised by small double-sided copper on substrates, cut into small squares placed vertically and soldered to the main and coupled lines. They have to be positioned precisely in order to obtain the best performance. In addition, they increase the coupling, so that their effect has to be included at the design stage. Some iteration may still be necessary.
A simple method to improve directivity is to make the coupler considerably shorter than the usual quarter-wave length. Couplers that are an eighth of a wavelength long provide abut 10 dB improvement in directivity. However, the coupling varies over the band of operation considerably, and is not acceptable except in very narrow-band and less stringent applications. In addition, the coupling gap has to be reduced to compensate for the lower coupling due to shortened coupling length.
U.S. Pat. No. 5,159,298 discloses a microstrip directional coupler which uses a single lumped element compensator, such as a capacitor or inductor, connected between the two conductors which define the primary and secondary transmission paths of the coupler, in order to improve directivity. However, the described directional coupler requires the use of non-planar cross-over fabrication techniques, and is therefore suitable only for microwave integrated circuit (MIC) and monolithic microwave integrated circuit (MMIC) applications.
It is an object of this invention to provide an improved microstrip directional coupler which overcomes or ameliorates one or more of the abovedescribed disadvantages.
The microstrip directional coupler of this invention uses inductive loading to achieve improved directivity.
The directional coupler is generally of conventional form having a dielectric substrate layer, a planar conductive layer on one side of the substrate layer which, in use, may serve as a ground plane, and first and second planar conductive strips laid on the other side of the substrate, the second conductive strip being electromagnetically coupled to the first conductive strip in use. The second (or coupled) conductive strip has a coupling section which is positioned side-by-side with the first conductive strip. The ends of the coupling section are connected respectively to a coupled port and an isolated port by transmission line sections of the conductive strip.
In one embodiment of the invention, the second conductive strip is inductively loaded to ground by a pair of inductive stubs, each connected to a respective one of the transmission line sections. Each inductive stub is a planar conductive strip on the substrate less than one quarter wavelength long at the operating frequency of the coupler, the end of the strip being connected through the substrate to the ground plane. The inductive stubs are designed to reflect a small amount of power from the coupled port to the isolated port to achieve phase cancellation, thereby improving directivity.
In a second embodiment, a pair of inductive stubs are connected to respective opposite ends of the coupling section of the second conductive strip. Another pair of inductive stubs are respectively connected to the first conductive strip at locations adjacent the ends of the coupling section of the second conductive strip. The stubs are short-circuited to the ground plane, and provide inductive loading for the even modes.
In a variation of the second embodiment, the even mode is inductively loaded in the middle of the coupled section by short-circuited inductive stubs connected respectively to the first and second conductive strips.
Although planar strip-like inductive stubs are preferred for ease of fabrication, lumped element inductances may alternatively be used.
In order that the invention may be more fully understood and put into practice, preferred embodiments thereof will now be described by way of example, with reference to the accompanying drawings.